The present invention relates to transistor design, and more particularly, to a novel bipolar junction transistor (BJT) on silicon on oxide (SOI).
A bipolar junction transistor is a three terminal electronic device commonly used in amplifying or switching applications. BJTs can be used both for high-power switch applications such as switched mode power supplies and for low-power switch applications such as logic gates. BJTs are also commonly used in vast numbers of products as amplifiers to provide current gain, voltage gain or sometimes both. From mobile phones to televisions, BJTs are used to provide sound reproduction, radio transmission, and signal processing.
A BJT is generally constructed of doped semiconductor material and its operation involves both electron and hole migration. A BJT can operate under different types of operation modes such as forward and reverse active mode, standby mode, cutoff and saturation mode depending on the biasing voltage applied.
Conventional bipolar transistors have a quasi-neutral base region of width WB. In a conventional NPN bipolar junction transistor (BJT), for example, the physical separation WE-C between the n+ emitter and n+ collector consists of three regions: the quasineutral intrinsic base of width WB, the base-emitter space-charge region of width WdBE, and the base-collector space-charge region of width WdBC. The relation between these regions is dictated by WE-C=WB+WdBE+WdBC.
During operation, WB is modulated by the bias voltages VBE and VBC, with WB typically remaining non-zero all the time. The collector current is controlled by the value of WB. Specifically, the collector current is inversely proportional to WB, and it varies with the applied emitter-base diode voltage VBE as exp(qVBE/kT). That is, the collector current varies in the form IC0×exp(qVBE/kT) where IC0 is more-or-less independent of VBE. At large VBE, the collector current can be quite large. At a small VBE, or at zero VBE, the collector current is extremely low, limited only by the collector-base diode background leakage current.
For low-voltage bipolar circuits, such as I2L (Integrated Injection Logic) and CBipolar (Complementary Bipolar, which is analogous to CMOS), achieving high switching speed typically requires the transistors to switch about equally fast in both forward-active mode (emitter-base forward biased) and in reverse-active mode (collector-base forward biased). The complementary SOI lateral bipolar transistors are ideal for low-voltage bipolar circuits. They switch equally fast in forward-active and reverse-active modes.